The present invention relates to infrared (IR) analysis of materials. It finds particular application in conjunction with transmission spectroscopy apparatus used during infrared analysis of liquid and solid phase materials by performing transmission spectroscopy analyses and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other applications.
While performing an infrared analysis transmission spectroscopy experiment of liquid and solid phase materials, infrared energy is passed through a thickness of a material being analyzed. The thickness of the material is typically no more than 100 microns for mid-infrared analysis and no more than 2 centimeters for near-infrared analysis. For strongly absorbing liquids such as aqueous based solutions, the thickness for mid-infrared analysis is typically much smaller (e.g., typically between about 10 microns and about 20 microns). Other than the typical need to use longer path lengths, analytical procedures for near-infrared analyses of liquids are very similar to those used for mid-infrared analyses. An exception relates to the typical need of near-infrared analyses to analyze many more samples to develop a robust method.
Sealed transmission cells are typically used to analyze liquids via transmission spectroscopy in the mid-infrared region whereas cuvettes are typically used in the near-infrared region. Transmission cells are typically sealed with an amalgam, gaskets, or o-rings. Such cells are typically filled using syringes and Luer-lok fittings, whereby a syringe containing a sample is attached to the input side Luer-lok fitting, and an empty syringe is attached to the exit side Luer-lok fitting. The syringes are simultaneously manipulated with “push-pull” actions to completely fill the cell without air bubbles. Cleaning the cell is accomplished in a similar fashion, whereby a solvent, instead of the sample, is placed in one syringe, and the solvent is then push-pulled into the empty syringe. An additional step of passing dry air through the cell further removes trace amounts of sample and solvent. Alternatively, small diameter tubing is used, whereby a liquid is made to flow through the cell by means of a pump or piston device. Cleaning cells via this arrangement occurs by using a valve to route a cleaning solvent through the cell, or more tediously, by disassembling the cell, cleaning the components, and re-assembling.
The typical analytical procedure when using sealed cells is: (1) establishing an instrument reference; (2) performing an analysis of the material of interest; and (3) executing a “method” using the information obtained in steps (1) and (2) to determine specific characteristics of the material of interest.
The precision, accuracy, and reliability of analyses depend upon many factors. In that regard, any unintended or unaccounted for changes in the three steps described above likely result in erroneous results. For example, if the reference cell is not sufficiently clean, an erroneous reference is established. If any substantive portion of the optical path is subjected to typical atmospheric changes of water vapor, carbon dioxide, and trace environmental gases, significant analytical measurement errors may result. If there is any change in the optical path of the cell, quantitative spectroscopic results are compromised. Therefore, any substantive unaccounted for changes between the development of the method and the execution of the method, and/or between the establishment of a reference and analyzing the sample, produces compromised results.
Several infrared transmission spectroscopy cells have been developed to address specific issues. Mid-infrared analysis of strongly absorbing liquids (e.g., liquids requiring path lengths less than about 20 microns) has been more routinely performed by attenuated total reflection (ATR) infrared analysis, as opposed to infrared transmission analysis. ATR cells have become widely utilized because of their ease of use.
Although infrared ATR analysis overcomes the time and difficulty of inserting and completely removing a material for analysis, the ATR technique has two problems that are not easily overcome. First, when using the ATR technique, infrared energy only penetrates a few microns into the material being analyzed. Therefore, ATR cannot universally be used to analyze any material that separates, or is in any way different in the bulk of the material as opposed to the surface of the material. Second, while the ATR technique allows for the effective path length to be increased by increasing the number of internal reflections, other factors, such as the ATR material's absorption, or the amount or placement of the material being analyzed become dominating negative factors.
For these and other reasons, there remains a need for having an easy to use and clean transmission spectroscopy sampling apparatus for performing infrared transmission spectroscopy analyses. Prior art, by virtue of the time, care, and difficulties associated with inserting and completely removing materials in transmission cells, has significantly limited the commercialization of infrared transmission spectroscopy analyses. In general, it is time consuming and difficult to make precise, accurate, and reliable quantitative analyses of liquids, pastes, and mulls by infrared transmission spectroscopy. The primary purpose of this invention is to reduce the time and difficulty of performing infrared transmission spectroscopy analyses while maintaining, if not improving, precision, accuracy, and reliability.
The present invention provides a new and improved apparatus and procedure which addresses the above-referenced problems.